Anal. Chem. 1985, 57, 1044-1049
1044
"1
GC/FTMS system is demonstrated by GC experiments in the absence of calibrant, which yield small mass errors, comparable to those obtained in the presence of calibrant. Although a large split prior to the mass spectrometer is required to maintain pressures necessary for optimum accurate mass results, the development of a differentially pumped dual cell should permit GC effluent to be directly introduced into the mass spectrometer, while required pressures are maintained.
"1
W
I . . .
020
60
I60
'
140
'
IBO
'
LITERATURE CITED
2ko
M U S lamu)
Figure 4. A plot of seven- and ten-component mixture data analogous to Figure 3 demonstrates the reliability of the linear relationship between mass and error determined in Figure 2. The data fall along the line reproduced from Flgure 3 for 64K data with a 1.14-MHz bandwidth.
it should be emphasized that wide band GC/FTMS experiments with low parts per million error should be routinely available on FTMS systems that operate at comparable magnetic fields for as few as 16K data points. To test the utility and reliability of the accurate mass data produced from the four-component mixture analysis, a sevenand ten-component mixture were run under similar conditions. Tables IV and V are summaries of measured masses and associated errors for experiments using a 1.14-MHz bandwidth and 64K data points. Both examples illustrate the low parts per million error mass measurement capabilities of FTMS. Average errors of 2.8 and 4.5 ppm were found for the two mixtures, respectively. A more demanding test of the reproducibility of the four-component work is demonstrated in Figure 4, which includes masses considerably higher than those analyzed previously. These data fall along the line of expected error values produced in Figure 3.
CONCLUSIONS If pressures are maintained in the mid-10-8 torr range, accurate mass measurement results with low part per million error for the wide band GC/FTMS measurements are routinely obtained. This indicates that the method has potential to permit incorporation of elemental composition information from accurate mass measurement data into algorithms for the identification of unknowns. In addition, the stability of the
(1) Kilburn, K. D.; Lewis, P. H.; Underwood, J. G.; Evans, S.; Holmes, J.; Dean, M. Anal. Chem. 1979, 57, 1420-1425. (2) Kimble, B. J.; McPherron, R. V.; Olsen, R. W.; Walls, F. C.; Burlingame, A. L. Adv. Mass Spectrom. 1980, 88, 873-877. (3) Holland, J. F.; Enke. C. G.; Allison, J.; Stutts, J. T.; Pinkston, J. D.; Newcome, B.; Watson, J. T. Anal. Ch8m. 1983, 55, 997A-1012A. (4) Gaskell, S. J.; Pike, A. W. Adv. Mass Spectrom. 1980, 88, 279-285. (5) Harvan, D. J.; Hass, J. R.; Schroeder, J. L.; Corbett, B. J. Anal. Ch8m. 1981. 53. 1755-1759. (6) Voyksner, R.' D.;'Hass, J. R.; Sonocoal, G. W.; Bursey, M. M Anal. Chem. 1983. 55. 744-749. (7) Schuetzle, D.: Prater, T. J.; Harney, T. M. A&. Mass Spectrom. 1980, 8B, 1082-1090. ( 8 ) "VG Analytlcal 7070EQ Product Details"; Publication No. VG PD 7070 EQ 882, VG Analytical, Ltd., Cheshire, England. (9) Ledford, E. B., Jr.; White, R. L.; Ghaderi, S.; Wilkins, C. L.; Gross, M. L. Anal. Chem. 1980, 52, 2450-2451. (IO) White, R. L.; Wilkins, C. L. Anal. Chem. 1982, 5 4 , 2443-2447. (11) Laude, D. A., Jr.; Brissey, G. M.; Ijames, C. F.; Brown, R. S.; Wilkins, C. L. Anal. Chem. 1984, 56, 1163-1168. (12) Francl. T. J.; Hunter, R. L.; McIver, R. T., Jr. Anal. Chem. 1983, 55, 2094-2096. (13) White, R. L.; Onylriuka, E. C.; Wilkins, C. L. Anal. Chern. 1983. 55, 359-343. (14) Ledford, E. B., Jr.; Ghaderl, S.; Whlte, R. L.; Spencer, R. B.; Kulkarni, P. S.; Wilkins, C. L.; Gross, M. L. Anal. Chem. 1980, 52, 463-488. (15) Francl, T. J.; Sherman, M. G.; Hunter, R. L.; Locke, M. J.; Bowers, W. D.; McIver, R. T., Jr. I n t . J . Mass Spectrom. Ion Proc. 1983, 5 4 , 189-199. (16) Jeffries, J. B.; Barlow, S. E.; Dunn, G. H. Int. J . Mass Spectrom. Ion Proc. 1983, 54, 169-187. (17) Ledford, E. B.; Rempel, D. L.; Gross, M. L. Int. J . Mass Spectrom. Ion Proc. 198311984, 55, 143-154. (18) Ledford, E. B.; Rernpel, D. L.; Gross, M. L. Anal. ch8m. 1984, 56, 2744-2748.
RECEIVED for review October 10,1984. Accepted January 15, 1985. Support of this research by the National Science Foundation through Grants CHE-80-18245 and CHE-82-17610 (a Department Research Instrument Grant) is gratefully acknowledged.
Postsearch Accurate Mass Measurement Filter for Gas ChromatographylInfrared Spectrometry/Mass Spectrometry and Gas Chromatography/Mass Spectrometry Data David A. Laude, Jr., Carolyn L. Johlman, John R. Cooper, and Charles L. Wilkins" Department of Chemistry, University of California, Riverside, California 92521
A new algorithm that takes advantage of the accurate ma88 measurement capabilities of Fourier transform mass spectrometry (FTMS) Is presented. I t Is demonstrated to yield increased reliability for interpretation of either GC/IR or GC/MS library search results. I n additlon, use of the aigorRhm improves the already highly reliable identtfication capabilities of a ilnked GC/IR/MS analysis system.
Despite the increases in mass spectral library sizes and refinements in library search algorithms (1-4), absolute 0003-2700185/0357-1044$01.5010
identification of unknowns by mass spectral search techniques remains suspect without incorporation of a second, independent, source of information for the analysis. The integration of infrared and mass spectrometric data offers two distinct advantages in the analysis of unknowns; (1) infrared data may permit unambiguous identification of structural isomers for which mass spectral data, alone, is insufficient; (2) use of two independent spectrometric techniques improves the reliability of compound identification because incorrect search data that results from poor spectral signal to noise (S/N) or an inadequate library is eliminated. The potential 0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
of GC/IR/MS data for mixture analysis has been well established with both separate (5, 6 ) and linked experiments (7-10).A further refinement of GC/IR/MS, the integration of molecular weight information obtained from chemical ionization MS data into the analysis scheme, has been demonstrated (11);if a comparison of IR and MS search results does not permit absolute compound identification, the use of molecular weight and IR search results provides a reliable alternate method for the analysis of unknowns. The combined analysis of GC/IR and GC/MS data has been performed for a variety of instrumentation with sector, quadrupole, and Fourier transform mass spectrometers used for such analyses. However, for a linked system in which all data for the analysis is obtained from a single chromatographic separation, the versatility of FTMS provides several advantages (12). The multichannel advantage permits rapid data acquisition with mass spectra acquired in a few milliseconds; this is especially important if the sampling requirements of capillary column GC/MS are to be met. In addition, electron impact and chemical ionization mass spectra may be acquired alternately during a single chromatographic separation with both mass spectral search and molecular weight information obtained. The recently demonstrated accurate mass measurement (AMM) capability of FTMS in the GC/MS mode permits the determination of molecular formula and elemental composition (13). In the present work, AMM data are incorporated in combined GC/IR/MS algorithms to further improve the reliability of identification procedures for unknowns. Although a variety of algorithms that utilize elemental composition data are possible, we have chosen to use the data in a post-library-search filter for both GC/IR and GC/MS data to eliminate results with incorrect molecular formulas. The unambiguous determination of molecular formula requires a typical mass error of less than 10 ppm for compounds with a molecular weight less than 300; an analysis of the sampling, experimental, and calibration requirements to obtain the requisite mass accuracy values has been conducted (13). Although smaller bandwidths and an increased number of data points improved the mass accuracy of a wide-band FTMS measurement, even in the absence of calibrant, 16K data collected over a mass range of 41 to >3000 amu (1.14 MHz) afforded an average of less than 10 ppm error at mass 250. Thus, experimental parameters for AMM GC/FTMS are not restrictive and are readily implemented in a GC/IR/MS experiment.
EXPERIMENTAL SECTION Accurate mass measurement spectra of 45 compounds were obtained by GC/FTMS measurements. In addition, IR spectra of the same compounds were acquired by independent GC/IR analysis. IR and MS library searches of the spectra for the 45 compounds were performed and then subjected to a postsearch AMM filter. GC and Sample Conditions for MS Data. A 5880-A Hewlett-Packard gas chromatograph with a 60 m X 0.323 mm i.d. J+W DB-5 bonded phase capillary column was used. Temperatures at the inlet, oven, and transfer line were maintained to permit elution of the sample at the void volume thus expediting the analysis. Typically, 0.05-0.1 p L of each compound was injected on the column with a 2O:l split, Postcolumn effluent was split with several hundred nanograms of each material introduced to the mass spectrometer. Mass Spectral Conditions. A Nicolet FTMS-1000 mass spectrometer (3.0-T magnet and 2.54 X 2.54 X 7.62 cms cell) was used for the analysis. Severe postcolumn restriction limited vacuum can pressures to the mid-lO-* torr range, sufficient for good accurate mass measurement results (13). A five-point calibration table was made from perfluorotributylamine prior to the GC/MS experiment; the calibrant was leaked through the volatile inlet at pressures comparable to those for eluting GC
1045
peaks. Actual GC/MS measurements were performed in the absence of calibrant with a single common calibration table that provided acceptable mass errors for several hours. One hundred coadded scans of 16K data with a 1.14-MHz bandwidth (low mass cutoff at m/z 41) were collected every 4.4 s and stored on a 300-Mbyte storage module disk. GC and Sample Conditions for IR Data. A Varian 3700 GC with a 25 m X 0.32 mm i.d. J+W DB-1 fused silica capillary column was used. Each compound was diluted in an appropriate solvent and injected on column (0.3p L injection volume). Oven temperature was maintained for isothermal operation at initial temperatures of 80-140 "C with transfer lines at 200 "C. Approximately 3 to 5 pg of each sample was introduced to the IR spectrometer. IR Conditions. A Nicolet 7199 FTIR with a 1.34 mm i.d. by 18 cm lightpipe (250 p L volume) was used. The spectrometer collected 204&poht interferograms which produced 8 cm-' spectra over a 750-4000 cm-' range. Seven interferograms were coadded and stored every 1.0 s. MS Data Processing: Magnitude mode mass spectra were obtained following a single zerofill and Fourier transformation. Frequency data for each spectral peak were calculated from the fit of a parabolic function to the most significant points on the peak. Accurate mass data were then obtained by interpolation of a mass-frequency calibration table (determined from an equation which accounts for the effects of space charge and trapping voltages) (14). A mass spectral search of a 32000 compound EPA/NIH library was performed for each compound with a modified Biemann algorithm (11). The top five search results were saved for later analysis in conjunction with accurate mass and IR data. IR Data Processing. The IR data were first apodized and then Fourier transformed (4096 points), phase corrected, ratioed to background spectra,and converted to 8 cm-l absorption spectra. With Nicolet-developed software, the SQ metric (15) was used to search the 3350-compound EPA vapor-phase library. The top ten matches for each of the 45 compounds were saved for further analysis.
RESULTS AND DISCUSSION Accurate mass measurement GC/FTMS analysis is routinely available provided mass spectrometer conditions are maintained a t low pressures (mid-lO-s torr) and sampling requirements are met. T o demonstrate the potential of the AMM algorithms, an average error of less than 10 ppm up to mass 250 is required; preliminary experiments to determine effects of bandwidth and number of data points on the mass error indicated that 16K data collected with a 1.14-MHz bandwidth (low mass cutoff a t 41 amu) was sufficient (13). Average error as a function of mass is plotted in Figure 1for the 45 compounds. The data conform to the predicted relationship between error and mass with an average error of about 6 PPm. A flow chart of the AMM algorithm logic applied to both GC/IR and GC/MS data is presented in Figure 2. Elemental composition and molecular formula data calculated from AMM spectra are used as post-IR and -MS filters to eliminate inconsistent search results. For each nominal mass peak of a top library search result, all possible elemental compositions and their corresponding exact mass values are determined. The calculated exact mass values are then compared with the experimental exact mass result for each mass fragment, and the error (in parts per million) is obtained. Generally, if an elemental composition is correct, the difference should be less than 10 ppm while incorrect compositions often produce errors of hundreds of parts per million. If the largest peak analyzed corresponds to the molecular species, then further calculations with fragment ions are redundant; however, all peaks are subjected to the AMM algorithm to ensure that in the absence of the molecular species the largest molecular formula is obtained from a maximum magnitude norm of smaller elemental composition fragments.
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
1046
-"
120
1
160
MASS (amu)
Figure 1. Average error for fragments (plotted as an average every 5-10 mass units) from mass spectra of 45 compounds. The data were acquired with 16K points over a 1.14-MHz bandwidth in the absence of calibrant. A weighted linear least squares plot of the data is also included which follows the predicted error-mass dependence calculated from previous GC/FTMS experiments (13).
Table I. Comparison of Various AMM Algorithms for the 45 Unknown Compounds number identified
algorithm
correct
incorrect
eliminated
IR IR/AMM MS MS/AMM IR/MS AMM IR/MS
32 33 26 30 32 35
13 8 19 12 0 0
0 4 0 3 13 10
MS search results are eliminated if a mass difference of less than 10 ppm at each fragment, indicative of the correct molecular formula, is not obtained. If the top five MS search results are eliminated by the algorithm, the compound is considered unidentified and a final calculation of the elemental compositions for each fragment is determined independent of the MS search results. All combinations of 12C,13C,H, 0, N, F, 36Cl,37Cl,79Br,81Br,P, Si, and S are considered in these elemental composition calculations. Following determination of the molecular formula from MS data, the IR search results are examined and eliminated if a subset of the search result molecular formula does not include the experimentally determined elemental composition. Again, if the top five IR search results are eliminated by the AMM algorithm, the unknown is considered to be unidentified. For the 45 compounds subjected to the analysis, correct molecular formulas were obtained for 35 compounds with a subset of the molecular formula obtained for the other ten compounds. Because no incorrect determinations of elemental compositions occurred, the MS sampling conditions selected (with an average error of 6 ppm) appeared to be sufficient for the analysis. Because elemental composition data were acquired from E1 spectra, correct molecular formula values often resulted from a maximum magnitude norm of the smaller fragment peaks. CI data would be expected to yield molecular formula information much more readily. Table I presents a summary of the 45-component search results for various combinations of the algorithms used for identification of the unknowns. In addition Table I1 lists the complete data for a selection of ten compounds in the 45component mixture to illustrate the potential variations in AMM algorithm results. The independent use of GC/IR or
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
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Table 111. Analysis of Aniline by MS/AMM MS Search Result search position
name
mol formula
pyrazole-4-carbonitrile aniline benzeneamine HC1 2-methylpyridine 4-methylpyridine AMM Algorithm
From unknown spectrum, peaks at m / z 93.056840 and 66.046760 search position
nom mass
elem comp
exact mass
error, ppm
1
93 66
C4H3N3
93.032684
C2N3
66.009222 66.021 789
-259.6 -568.4
C3H2N2
CBHI C5H6
93
2
66
result eliminated
-378.1
93.057 818
10.5
66.046 923
2.5
correct M.F.
MS/AMM Search Result search position
name
1 2 3 4
aniline benzeneamine HC1 2-methylpyridine 4-methylpyridine
DETERMINE SEARCH MOLECULAR FORMULA
DETERMINE s E m c n FRAGMENT FORMUAS
FRAGMENT FORMULA EXACT MASSES
7 I DENTlFlCATlON
9 SE~~PJ%%’<
Figure 2. Flow
ELIMINATE SEARCH RESULT
0
chart of the AMM algorithm for G W I R and GWMS
data. GC/MS search results is found to be the least reliable of the identification methods. Although proper identification is made for 73% of the IR data and 57% of the MS data, no method is available to substantiate the identification of an unknown; such information is crucial especially for spectra with poor SIN or for compounds not found in the library. The molecular formula data, in combination with either IR or MS search results, provide a mechanism to either promote search results to a proper match with the data or invalidate the search results completely. As seen from Table I, the number of correct identifications increases for the MSIAMM and IR/AMM algorithms by four and one, respectively. The number of incorrect assignments is also decreased with the elimination of four and three search results for each AMM algorithm. Particular examples of the AMM algorithms from analysis of the 45 compounds are presented in detail in Tables 111-V to clarify the procedure. In Table 111, the MS/AMM algorithm is used to correctly identify, from MS search data, the correct compound, aniline. An.MS search alone places aniline behind the number one match, pyrazole-4-carbonitrile.The
mol formula C6H7N
CeH7NClH C6H7N
AMM algorithm first requires determination of all possible empirical formulas at the nominal masses of 93 and 66 found in both the library and unknown spectra. Accurate mass calculations for the elemental compositions of the rn/z 93 and 66 ions of pyrazole-4-carbonitrile produce part per million differences of several hundred, sufficient to eliminate the compound from the search list. Similar analysis of the elemental compositions of the aniline nominal mass fragments yields only small discrepancies. In Table IV, the MS search results for the unknown, 2,2,2-trichloroethanol, are incorrect because of poor spectral SIN. Application of the AMM algorithm to the top MS search result produces a large parts per million difference between the library and unknown spectrum. The other four MS search results listed provide similarly poor mass measurement data and are eliminated. If the top search results are all eliminated, then an independent calculation of elemental composition which considers all combinations of the major isotopes of nine elements, is produced. For the 119 and 115 exact mass peaks of the unknown, elemental compositions of CCl, and C2H30C12 are determined. The maximum magnitude norm of these results yields the correct molecular formula despite the lack of a molecular ion. This molecular formula information can be used subsequently in the IR/AMM algorithm. In Table V, the IR/AMM algorithm is utilized to provide correct identification of the unknown, bromoform. The top five IR search results include bromoform in the second search position prior to the postsearch filter. Because a molecular formula has already been determined from the MS data, elimination of incorrect IR search results is efficiently performed by a comparison of the molecular formulas. For this case the number one hit is eliminated along with two other of the top five search results and the postsearch result correctly places bromoform in the first position. The AMM algorithm as applied to the individual IR and MS data alone is imperfect; 8 and 12 cdhpounds, respectively, are still incorrectly identified. This result is expected because of the inability of the algorithm to eliminate structural isomers. However, the ease with which accurate mass GC/FTMS data are obtained recommends the incorporation of accurate mass data in independent GC/MS experiments to facilitate the
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
1048
Table IV. Analysis of 2,2,2-Trichloroethanol by MS/AMM MS Search Result search position
name
mol formula
fluorotrimethylsilane ethylfluorodimethylsilane
1,l-hydrazinedisulfinic acid, 2,2-dimethyl ester 2- bromo-1-chloropropane boron trifluoride AMM Algorithm From unknown spectrum, peaks at m / z 60.98460,95.95253, 114.95372, and 118.90455 search position
nom mass
elem comp
61
1
exact mass ( m / z )
CPH5Si C3Ha5C1 C3H&lSi no exact mass
96 115, 119
61.011175 61.045326 96.040860 values within 1000 ppm
error, ppm
result
435 996 920
eliminated
2-5
eliminated Molecular Formula Determination
nom mass
elem comp
119 115
CzHSOC12
exact mass (calcd)
exact mass (exptl)
error
118.90360 114.953 14
118.904 55 114.95372
4.9
CC13
7.9
Molecular Formula from Maximum Magnitude Norm C2H30C13 = 2C + 3H + 10 + 3C1 Table V. Analysis of Bromoform by IR/AMM IR Search Result
position
name
1 2
trifluoromethane bromoform 1,l,a-tribromomethane cup-dibromotoluene 1,l-difluoromethane AMM Algorithm
3 4
5
mol formula CHF3 CHBr,
Figure 3. Block diagram of the Integrated GC/IR/MS experiment which Incorporates E1 and CI spectra whh accurate mass measurement data.
C2H3Br3
C7H6Br2
C2H4F2
From unknown spectrum, peak at m/z 253.756689 MS search position
nom mass elem comp
1
254
CHBr,
exact mass
error, ppm
253.758736
8.1
Comparison of CHBr, with IR Search Molecular Formulas Eliminates 1, CHF,; 4, C7H6Br2; and 5, CzH4F2
IR/AMM Search Result search position 1 2
name bromoform
lJ.2-tribromoethane
mol formula
CHBr, CeHnBr2
correct identification of unknowns. If both IR and MS data are available, the complementary search results should be compared to obtain correct identification from among isomers. Table I presents the results of IR and MS search comparisons with both filtered and unfiltered AMM data. T o distinguish between multiple matches of IR and MS data, a correlation value, the multiplicand of IR and MS search positions, is determined and the smallest correlation value is assumed to be the correct result. Of the 45 compounds, 32 are correctly identified by IR/MS matches, 13 are eliminated,
and none is incorrectly identified. The use of the AMM algorithm with the IR/MS comparison improves the number of compounds identified to 35 with 10 eliminated. 2,2,2-Trichloroethanol is correctly identified based upon the IR/AMM algorithm alone because the mass spectral search results are invalidated as demonstrated in Table IV. 7,7-Dichlorobicyclo[4.l.0]heptaneand p-fluorobenzaldehyde are added to the list of correctly identified compounds despite their absence from the IR library because, although the IR/MS algorithm alone cannot produce a correct identification, the AMM algorithm permits identification based solely upon MS/AMM data. In addition, the correlation value for 9 of the 11 compounds which were not top IR and MS search results prior to use of the AMM algorithm was improved by the elimination of incorrect IR and MS search data. CONCLUSION The utility of several complementary sources of information, including infrared search results (5-IO),molecular weight data from CI mass spectra ( I I ) , and now elemental composition data from AMM mass spectra, has been demonstrated to improve the reliability of interpretation of mass spectral search results for the conventional GC/MS experiment. Independent collection and analysis of this information from discrete chromatographic experiments, however, would require exhaustive data acquisition and processing times. A single chromatographic experiment that produces IR and MS results in addition to molecular weight and molecular formula data would be far more efficient; Figure 3 depicts the logic of an
Anal. Chem. 1985, 57, 1049-1056
integrated GC/IR/MS system and indicates the information available. The versatility of FTMS recommends it for the approach described, as both accurate mass and chemical ionization data, in addition to E1 mass spectra, are potentially available from a single analysis. Although pressure requirementa differ markedly for the collection of CI and AMM data, the use of pulsed values (16)to provide momentary CI reagent gas pressures, in addition to a dual-differentially pumped trapped ion cell to maintain constant pressure for AMM, should provide the means for performing the integrated measurement.
LITERATURE CITED (1) Hertz, H. S.; Hltes, R. A.; Elernann, K. Anal. Chem. 1971, 43, 681. (2) McLafferty, F. W.; Hertel, R. H.; Villwock, R. D. Org. Mass. Spectrom 1974, 9 , 690. (3) Pesyna. G. M.; Venkataraghavan, R.; Dayrlnger. H. 0.; McLafferty, F. W. Anal. Chem. 1978, 48, 1362. (4) Henneberg. D. Adv. Mass Spectrom. 1980, 68, 1511. (5) Schafer, K. H.; Hayes, T. L.; Erasch, J. W.; Jakobsen, R. J. Anal. Chem. 1084, 56,237-240. (6) Chiu, K. S.; Blernann, K.; Krlshnan, K.; Hill, S. L. Anal. Chem. 1084, 56, 1610-1615.
1049
(7) Wilkins, C. L.; Glss, G. N.; White, R. L.; Erlssey, G. M.; Onylrluka, E. C. Anal. Chem. 1982, 54,2260-2264. (8) Wllkins, C. L.; Giss, G. N.; Brlssey, G. M.; Steiner, S. Anal. Chem. 1081, 53, 113-117. (9) Crawford, R.; Hirschfeld. J.; Sanborn, R. H.; Wang, C. M. Anal. Chem. 1982, 54,817-820. (IO) Wilklns, C. L.; Glss, 0. N.; Erlssey, G. M.; Ijarnes, C.; Steiner, S. 1983 Pacific Conference on Chemistry and Spectroscopy, Oct 27, 1983 Paper No. 267. (11) Laude, D. A., Jr.; Erlssey, G. M.; Ijarnes, C. F.; Brown, R. S.; Wilklns, C. L. Anal. Chem. 1984, 56, 1163-1168. (12) Ledford, E. E., Jr.; Ghaderi, S.; White, R. L.; Spencer, R. B.; Kulkarni. P. S.; Wllkins, C. L.; Grass, M. L. Anal. Chem. 1980, 52, 463-468. (13) Johlman, C. L.; Laude, D. A., Jr.; Wilkins, C. L. Anal. Chem., preceding paper In this Issue. (14) Ledford, E. E., Jr.; Rempel, D. L.; Gross, M. L. Anal. Chem. 1984, 56, 2744-2748 -. . . -. .-. (15) Lowry, S. R.; Huppler, D. S. Anal. Chem. 1981, 53,889-893. (16) Sack, T. M.; Gross, M. L. Anal. Chem. 1983, 55,2419-2421.
RECEIVED for review October 10,1984. Accepted January 15, 1985. Support from the National Science Foundation under NSF Grants CHE-80-18245 and CHE-82-08073 and a Department Research Instrument Grant, CHE-82-17610, is gratefully acknowledged.
A Numerical Method for Extracting Mass Spectra from Gas ChromatographyIMass Spectrometry Data Arrays Martin D. King*’ and Graham S. King2 Bernard Baron Memorial Research Laboratories, Queen Charlotte’s Hospital, London W 6 OXG, United Kingdom
A numerical method Is descrlbed for extracting single-component mass spectra from GWMS data arrays containing poorly separated spectral and chromatographic informatlon. The procedure Is an extension of the 1981 least-squares method of Knorr, Thorshelm, and Harris for decomposlng the data array into matrices containing the resolved spectra and chromatogram profiles. The modifications include an exponential down-scan lntenslty correction, a routine for reconstructing saturated mass spectra, and a background subtractlon facility. Furthermore, the requirement for previously obtained chromatographic Information is eliminated through the use of a nonilnear unconstrained optimization routine to determine all the parameters that characterize the chromatographlc behavior.
The capability of any chromatographic analytical technique to detect and identify poorly separated components is, in general, considerably enhanced through the use of a multichannel detection system. A mass spectrometer, when interfaced to a chromatographic apparatus, can function as a particularly powerful multichannel detector, and GC/MS is now a widely used analytical technique, particularly in the pharmaceutical industry and in clinical laboratories. A number of mathematical methods have been developed for processing multichannel chromatographic data arrays and these have been used to tackle GC/MS problems. Dromey *Present address: Abteilung Spektroskopie,
Max Planck I n s t i t u t
fur Biophysikalische Chemie, D-3400Gottingen, Federal Republic
of Germany. Present address: M e t a b o l i s m a n d Residue Section, Protection, Jeallot’s Hill, Backnell, Berkshire, U.K.
I.C.I. P l a n t
0003-2700/85/0357-1049$01.50/0
et al. (I) have adopted a procedure in which the data matrix is scanned for the sharpest mass chromatograms. The ions giving rise to these narrow peaks are assumed to be unique to one of the overlapping components and their chromatograms are used to define model peak shapes and hence to resolve the overlapping components. Other researchers have developed routines which rely more heavily on numerical methods. Rather elegant treatments have been suggested by Sharaf and Kowalski (2) who use factor analysis to achieve the separation and Knorr, Thorsheim, and Harris (3) (hereafter referred to as KTH) who have developed a least-squares procedure. In the present paper we outline a number of modifications that have been incorporated into the method described by KTH in order to tailor the procedure to the needs of our laboratory, which is principally concerned with metabolic profile analysis. These modifications include a cubic-spline interpolation procedure to correct for the exponential downscan time delay, a method for reconstructing saturated mass spectra, and a modification to deal with background interference. KTH adopt a procedure in which the chromatographic “response” is determined by running a series of standards, after which the number of theoretical plates and a decay time constant are calculated. This information is required to perform the decomposition. While this approach has its advantages, for routine use in a busy clinical laboratory the need to run many calibration standards was considered unacceptable in terms of the work it would entail. Furthermore, a large number of compounds are present within a single metabolic profile, and these exhibit a wide variety of chromatographic characteristics, making the calibration almost impossible. We therefore sought to eliminate the need to run calibration standards by using a nonlinear unconstrained optimization routine to extract from the GC/MS data matrix 0 1985 American Chemical Society
